Wavelength-tunable amplified optical splitter

ABSTRACT

A signal may be split by a splitter into a plurality of output signals. Each of these output signals may then be amplified. Amplified spontaneous emission noise may be removed using a tunable filter for each of the signal outputs. As a result, an output signal may be provided with greater power so that, in some embodiments, a single split signal may be utilized to service more end users.

BACKGROUND

This invention relates generally to optical networks and, particularly,optical networks that use optical power splitters.

In an optical network, a signal may be transmitted over an opticalfiber. The signal may include a plurality of channels, each of adifferent wavelength. In order to multiplex the different channels ontothe fiber, a multiplexer may be used. A demultiplexer is used toseparate the multiplexed channels at a destination.

A power splitter may divide a channel into a plurality of distinctoutputs. A signal, containing a single channel or multiple channels, maybe divided by a splitter and delivered to several differentdestinations.

Amplification is required to compensate for propagation losses and lossof power of the signals due to splitting. The amplification of theoptical signal is usually provided by erbium-doped fiber amplifiers.

Currently, there is particular demand for optical splitter devices foruse in fiber-to-the-curb (FTTC) and fiber-to-the-home (FTTH)communication networks. These splitter devices facilitate thedistribution of a common signal to multiple customers. However, aconventional splitter severely limits the transmission link length andthe number of customers due to the natural signal loss associated withevery splitting function.

Erbium-doped amplifiers can be used to compensate for such losses,significantly increasing the number of customers that receive the samesignal. However, erbium-doped amplifiers are too expensive for thislow-cost application. Also the broadband amplified spontaneous emission(ASE) noise generated in the amplifier degrades the signal-to-noiseratio, posing a limit on the number of customers serviced by the splitsignal.

Thus, there is a need for better ways to provide amplified splitting inoptical networks.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic depiction of one embodiment of the presentinvention;

FIG. 2 is a more detailed depiction of the embodiment shown in FIG. 1 inaccordance with one embodiment of the present invention;

FIG. 3 shows a hypothetical input to the splitter 12 shown in FIG. 1, inaccordance with one embodiment of the present invention;

FIG. 4 shows a hypothetical output from the splitter 12 in accordancewith one embodiment of the present invention;

FIG. 5 shows a hypothetical output from the amplification gain block 14in FIG. 1 in accordance with one embodiment of the present invention;

FIG. 6 shows a hypothetical output from the tunable filter chip 16 shownin FIG. 1 in accordance with one embodiment of the present invention;and

FIG. 7 is a system schematic depiction in accordance with one embodimentof the present invention.

DETAILED DESCRIPTION

Referring to FIG. 1, a 1×N splitter 12 receives an input signal 18 whichmay be an optical multiplexed signal. The splitter 12 splits the inputsignal 18 into N output signals. For example, an input signal, as shownin FIG. 3, may be split to produce a plurality of output signals of thetype shown in FIG. 4. Each split signal, as shown in FIG. 4, has thesame peak wavelength as the input signal 18, but the amplitude of thatpeak may be substantially diminished compared to the amplitude of theinput signal 18.

A variety of splitters 12 may be utilized, including a cascadedY-junction splitter and a multi-mode interference splitter.

The split output signals from the splitter 12 are then amplified by theN-channel amplification gain block 14. The amplification gain block 14may use pump lasers and erbium-doped waveguides in one embodiment. Theoutput from the gain block 14 in one hypothetical example is shown inFIG. 5. While the peak power is now higher, a noise floor has beencreated as a result of amplified spontaneous emission (ASE) from theamplifier.

The split signals from the gain block 14 may then be subjected to anN-channel, tunable filter 16 in accordance with one embodiment of thepresent invention. The filter 16 removes the noise floor resulting inthe hypothetical output signal shown in FIG. 6. The filter 16 may, forexample, be a thermo-optically tuned waveguide Bragg grating pair thatis written using ultraviolet light on an integrated Michelsoninterferometer. In such case each of the Bragg gratings 29 is heated toa certain temperature to tune the reflected band of resonant wavelengthsto correspond to the wavelength of the peak amplitude.

As another example, the reflected light from a single reflective Bragggrating can be separated from incident light using an optical circulator(not shown). The circulator passes the input light and outputs the lightreflected by the tunable Bragg grating. As still another example, asingle transmissive tunable Bragg grating may be used to pass thedesired band of resonant wavelengths corresponding to the peakamplitude.

The structure shown in FIG. 1 may be made using a monolithic integrationapproach with all three functional blocks, 12, 14, and 16 fabricated ona single planar waveguide optical chip. Alternatively, in the hybridapproach, functional blocks may be fabricated in separate chips and thendirectly attached in a multi-chip module format. In still anotheralternative, a fiber integration approach may be used in which thefunctional blocks are fabricated and packaged separately and theninterconnected by way of optical fibers.

In some embodiments, the use of the tunable filter 16 may significantlyincrease the number of end points or customers accessible by a commonnetwork node. This may mitigate one of the most severe bottlenecks inFTTC/FTTH communication systems, namely, the restriction of the linklength and the number of customers for a common signal due to lossesassociated with splitting. Furthermore, in some embodiments, the usermay have less ASE noise, thereby improving the bit-error rate of thetransmission system.

Referring to FIG. 2, an embodiment is illustrated in which the splitter12 is implemented by a series of 1×2 Y-junction splitters 24. An inputpump 22 may be provided to each split signal from the splitter 12 asindicated at 22 a. The amplified signal line 26 exits the N-channelamplification gain block 14 and may go to a Michelson interferometer 28in one embodiment. One of the arms of the interferometer 28 may providethe output signal 20.

A thermally heated Bragg grating 29 may be provided in each of two armsof the Michelson interferometer 28. These Bragg gratings 29 act as anoptical filter to select one or more desired bands of wavelengths toform the output 20. In one embodiment, the Bragg grating pairs 29 filtera desired band of resonant wavelengths by reflecting that band to becomethe output signal 20. If the wavelength of the reflected band is tunedby heating, using the heaters 40, to correspond to the peak amplitude(see the pass band A in FIG. 5), the noise floor (FIG. 5) may be removed(FIG. 6). In one embodiment, the heaters 40 may be micro-heaters thatheat using electrical resistance.

Referring to FIG. 7, an optical system may include a multiplexer 30 thatmultiplexes a N number of channels 1 through N. Those signals may thenbe amplified by an amplifier 32 which may use erbium doping. A switch 34may be used to switch different signals before demultiplexing at thedemultiplexer 36. Each demultiplexer signal may go to a desireddestination 38. Alternatively, the tunable splitter 10 may be utilizedto further split the signal to increase the number of end users that canbe serviced by the same demultiplexed output signal.

While the present invention has been described with respect to a limitednumber of embodiments, those skilled in the art will appreciate numerousmodifications and variations therefrom. It is intended that the appendedclaims cover all such modifications and variations as fall within thetrue spirit and scope of this present invention.

1. An optical splitter comprising: a splitter section to create at leasttwo output signals from a single input signal; an amplifier to amplifyeach of the at least two output signals from the splitter section; and atunable filter configured to filter amplified spontaneous emission noisefrom each of said at least two output signals.
 2. The splitter of claim1 wherein said tunable filter includes a Bragg grating.
 3. The splitterof claim 2 wherein said tunable filter includes a pair of Bragggratings.
 4. The splitter of claim 3 wherein the tunable filter includesa Michelson interferometer including a pair of reflective Bragggratings.
 5. The splitter of claim 1 including a reflective Bragggrating.
 6. The splitter of claim 1 wherein said splitter sectionincludes a Y-junction splitter.
 7. The splitter of claim 1 wherein saidtunable filter passes a band of wavelengths that includes the peakwavelength of the input signal.
 8. The splitter of claim 1 including atunable filter that is tunable through a thermo-optic effect.
 9. Thesplitter of claim 8 wherein said tunable filter includes a Bragg gratingwith a heater.
 10. An optical splitter comprising: a splitter section tocreate at least two output signals from a single input signal; anamplifier to amplify each of the at least two output signals from thesplitter section; and a tunable filter including at least one Bragggrating configured to filter amplified spontaneous emission noise fromeach of said at least two output signals.
 11. The splitter of claim 10wherein said filter includes a Michelson interferometer including a pairof Bragg gratings.
 12. The splitter of claim 10 including a heater toheat said Bragg grating to tune the pass band of the filter.
 13. Thesplitter of claim 10 including a tuner to allow the pass band to betuned.
 14. The splitter of claim 10 wherein said Bragg grating passeswavelengths within the pass band.
 15. The splitter of claim 10 whereinsaid splitter section includes a Y-junction splitter.